Saving mobile data

Just back from Zambia. On the SPAKA bundles you can get (on MTN) there
is a good minutes allowance, but not a lot of data (e.g. 50 MB).
Similarly, on Emirates, you get 20 MB of WiFi free. So how do you make
that go as far as possible?

Install a firewall, and disable all apps, except the ones you really need, such as mail, your messenger app, and maybe a browser. Two firewalls: